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3. TECHNICAL BRIEF
A. RF Front End
RF front end consists of Antenna Switch(FL401), dual band LNAs integrated in transceiver(U401). The Received RF signals (GSM 925MHz � 960MHz, DCS 1805MHz � 1880MHz) are fed into the antenna or mobile switch. An antenna matching circuit is between the antenna and the mobile switch. The Antenna Switch (FL401) is used to control the Rx and TX paths. And, the input signals VC1 and VC2 of a FL401 are connected to DCSSEL(GPO_9) and GSMSEL(GPO_11) ports of U101 to switch either TX or RX path on. When the RX path is turned on, the received RF signal then feeds either Rx _900_RF or RX_1800_RF path controlled by GSM-RX and DCS-RX respectively. This Rx_900_RF path contains one SAW filter, followed after the Antenna Switch (FL401), to filter any unwanted signal apart from the DCS RX band. And, the RX_1800_RF path is the same case. The logic and current for Antenna Switch is given below Table 3-1. Table 3-1 The logic and current
VC1 GSM TX DCS TX GSM/DCS RX 0V 2.7 V 0V
VC2 2.7 V 0V 0V
Current 10.0 mA max 10.0 mA max <0.1 mA
These two paths are then connected to the LNAGSMIN (#11) and LNADCSIN (#13) of CX74017 (U401), respectively. A low-noise bipolar RF amplifier, contained within the U401, amplifies the RF signal. The RF signals from the front-end pass to the receiver mixers within the U401 device.
B. Demodulator and baseband processing
In direct conversion receiver there is only one mixer that is down-converting received RF signal to BB signal directly. The gain of the mixer is 40dB at high gain mode and 22dB at low gain mode. The Rx gain setting is done in the AGC algorithm. The nominal gain of the receiver is set as a function of the expected signal strength at the antenna input so that a desired level is reached at the Rx I/Q. 7 blocks in the receiver chain have variable gains, LNA, Mixer, LPF1, VGA1, gmC Filter, Auxiliary gain control and VGA2. The gain settings can be adjustable via 3-wire bus control lines. The baseband signals pass via integrated low-pass filters to the baseband A/D converters. The remainder of the channel filtering is performed by the baseband chipset. The demodulator contains switches to maintain the sense of the baseband I/Q outputs with respect to the incoming RF signal on both GSM900 and DCS1800.
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